The invention relates to a method for joining two metal plates together by welding and to a welded structure.
It is usual to use a large spherical tank for transportation and storing of liquified natural gas (LNG). The temperature of LNG is about -163.degree. C., which places special demands on the choice of material for an LNG-tank and on the technique used for producing such a tank. Such a tank is preferably made of welded aluminum plates because the extremely low temperature of the contents of the tank does not have a deleterious effect on the strength properties of aluminum. The material of weldable plates suitable for the production of an LNG-tank is, for instance, according to the DIN-standard AlMg4, 5Mn, which corresponds to the U.S. ASTM-standard plate A-5083. As used herein, in relation to the material of weldable plates, the term aluminum includes an alloy of aluminum. The thickness of the aluminum plates used for manufacturing LNG-tanks is usually from about 25 mm to about 60 mm. The plates are arranged in abutting relationship and meet at a weld groove. The diameter of a typical spherical LNG tank is from 30 m to 40 m, and there are typically several hundred meters of joints to be welded in one such tank.
In construction of a spherical LNG-tank by welding aluminum plates, it is frequently necessary that a joint that is being welded be oriented vertically, whereby the welding takes place from below upwards. Due to the orientation of the weld groove, there is a tendency for the molten metal (or weld pool) to run out of the weld groove due to gravity. This tendency appears of course most strongly when the weld groove extends vertically, but may also appear to some extent, complicating the welding operation, when the angle of the weld groove to the horizontal is as small as 30.degree..
According to the known art of vertical welding, a cooled weld pool support device is used. The weld pool support device comprises a dam member that is moved upwards in the direction of the weld groove as the welding proceeds in the same upward direction. When welding steel, a piece of copper has usually been used as the dam member in a weld pool support device, but when welding aluminum difficulties have occurred, because a copper dam member readily adheres to molten aluminum. For solving this problem it has been suggested to use as the dam member a piece of graphite, see U.S. Pat. No. 3,585,343 or Welding Journal, July 1973, pages 440-448. However, a graphite dam member wears out far too rapidly to be satisfactory solution to the problem.